If the party has well-planned for their win, that's a good thing and they should be rewarded for it.
At the same time, if you do want to spice things up, there's always the "no plan survives contact with the enemy" and "things don't go smooth". That is, some unexpected, unaccounted for action by someone else can throw a monkeywrench into the plans - not countermanding the player's plans, but threatening to complicate things if they aren't dealt with appropriately (many heist movies use this subplot to create tension and drama to an otherwise "perfect" plan - such as Ocean's 11).
Bad weather hits unexpectedly, threatening to delay the party from arriving on time. Someone at court offends the duke and he orders everyone out and the party has to find another way in. Someone overhears part or all of the player's plan when they accidentally walk in on the characters, and has to be dealt with. One of the duke's sicophants challenges a PC to a duel, suspecting treachery.
Even if you don't go with any of the above, there's always the fallout what happens AFTER the duke is outed. Likely, he has supporters or allies who will not take kindly to the duke being offed, and may not believe the PC's story. There may be a spouse or offspring that attempt to avenge the duke's death - righteously or not. Other elf-haters may take up the duke's banner, either avoiding the PCs or seeking them out. Perhaps even elves may become wrathful learning the dukedom was willing to employ a demon to wipe them out, and the PCs may have to deal with stopping a war against or reprisals by the elves.